A common problem found in high speed communication systems is inter-symbol interference (ISI) that results from multipath propagation. Multipath propagation is caused when delayed copies of the same signal arrive at the receiver. Delayed signals result from signal reflections from both terrain features such as trees, hills, and/or mountains and objects such as people, vehicles, and/or buildings. Because the signal reflection travels along a longer path, the reflected signals take more time to reach the receiver. The resulting delayed copies of the signal interfere with each other and with the possible direct signal causing ISI. Multi-carrier communication methods overcome ISI by subdividing the allocated bandwidth into smaller frequency sub-bands or sub-carriers. At each sub-carrier (sub-band), the data is transmitted using long symbol durations compared to the time delay between reflected signals. As a result, the impact of ISI is reduced.
Multi-carrier communication is proposed for the further evolution of third generation (3G) wireless systems including Evolved Universal Terrestrial Radio Access Network (EUTRAN), IEEE802.15.3a systems with MB-OFDM, fourth generation (4G) wireless systems, non-cellular communication systems, and short range communication systems based on multi-carrier modulation. Orthogonal Frequency Division Multiplexing (OFDM) is an implementation of a multi-carrier communication system in which the frequency sub-bands overlap. In an OFDM system, various modulation schemes may be used to modulate the data onto each sub-carrier. The incoming serial data is first converted from serial to parallel and grouped into “x” bits each to form a complex number. The number “x” determines a constellation of the corresponding sub-carrier. In communications terminology, a constellation is a pattern that represents the possible states of a carrier wave, each of which is associated with a particular bit combination. A constellation shows the number of states that can be recognized as unique changes in a communications signal, and thus represents the maximum number of bits that can be encoded in a single change. The modulation scheme in an OFDM system can be selected based on the requirement of power or of spectrum efficiency or based on other transmission considerations. For example, Binary Phase-Shift Keying (BPSK), Quadrature Phase-Shift Keying (QPSK), 16 Quadrature Amplitude Modulation (QAM), 32 QAM modulation methods may be used for a sub-carrier modulation. Error correction coding may also be applied to each sub-carrier.
Transmission parameters generally are constrained by an acceptable bit error rate and by a Signal to Interference plus Noise Ratio (SINR) of the signal. To provide for higher data throughput in wireless communication systems, Adaptive Modulation and Coding (AMC) methods are used in which both the modulation complexity and channel coding complexity are varied in response to changing channel conditions. In some communication systems such as systems implementing High Speed Downlink Packet Access (HSDPA), a number of channelization codes can also be varied in response to changing channel conditions. Varying modulation complexity means varying the number of bits that are communicated per symbol where a given modulation complexity provides a constellation of symbols with each symbol used to convey a bit string. The greater the number of symbols in the constellation, the longer the bit string conveyed by each symbol. Varying the channel coding complexity means, for example, varying the amount of redundancy included in forward error correction of the data to be transmitted. Varying the number of channelization codes means changing the number of channels multiplexed together by use of a code tree. Thus, AMC provides for the selection of a modulation method (comprising a matrix or a vector modulation), a constellation, a coding rate, a number of channelization codes, and/or an error correction code for each sub-carrier or cluster of sub-carriers in a multi-carrier system to allow adjustment of the transmission parameters thereby accommodating for changes in the channel characteristics over time. Thus, if a change in channel characteristics results in a lower SINR, the modulation level may be reduced (for example, from 16-QAM to QPSK or QPSK to BPSK) or the coding rate may be improved (for example, from ⅓ to ⅕) to maintain an acceptable bit error rate.
An example communication system includes a base station that transmits a signal to a remote unit. In a communication system implementing AMC, the base station and the remote unit must be synchronized with respect to the transmission parameters. In a Transmission Format Indication (TFI) system, the base station determines the transmission format based on measurements and possibly on feedback from the remote unit concerning the signal quality. The format used may be indicated to the remote unit in a TFI field, for example, in a common channel or in a channel header. In a feedback system, the remote unit selects the suitable transmission formats for the signal transmission and feeds the information back to the base station.
In either a TFI system or a FeedBack (TFI/FB) system, the remote unit may determine a channel quality when the remote unit receives a frame of data, for example, based on the SINR of the channel. In a TFI system, the remote unit sends a signal back to the base station reporting the channel quality. Using the channel quality report received from the remote unit, the base station calculates a set of transmission parameters that the base station will use in its next transmission of data. However, the base station must first send the set of new transmission parameters to the remote unit using the previous transmission parameters. Alternatively, a separate communication channel, often referred to as a control channel, may be established between the base station and the remote unit, and the information related to the new transmission parameters is sent on this control channel. The parameters of the control channel may be predefined, or they may change according to the perceived channel quality The remote unit receives the set of new transmission parameters using the previous transmission parameters. The remote unit then synchronizes subsequent frames of data using the new or predefined transmission parameters. In a feedback system, the remote unit calculates a set of transmission parameters for the base station to use, or a limited set of channel quality measurements that the base station may use to determine transmission parameters to use, in its next transmission of data. The remote unit sends the set of new transmission parameters to the base station using a feedback channel. The base station sends subsequent signals using the set of new transmission parameters. The system may be defined in a way that the base station may choose to use the transmission parameters suggested by the remote unit directly, or the base station may select other transmission parameter, if e.g. the traffic flow or the kind of traffic would require a different kind of transmission than the remote unit has assumed when calculating the set of transmission parameters. In such case, a feedback system and TFI system are used simultaneously.
In a multi-carrier system, AMC provides for the adjustment of transmission parameters for each sub-carrier or cluster of sub-carriers. In communication systems employing many sub-carriers, such as those that employ OFDM, the channel quality may vary with the frequency of each sub-carrier. OFDM systems can use thousands of sub-carriers. As a result, the transmission of information describing the channel quality and the set of transmission parameters for each sub-carrier requires significant overhead that may result in an efficiency reduction. Additionally, the AMC set that includes the possible transmission modes may be large typically because there are multiple rates and/or modulation alternatives. For example, in Multiple Input Single Output (MISO) systems and in Multiple Input Multiple Output (MIMO) systems, there are additional alternatives that relate to diversity and spatial/code multiplexing options. Thus, with many coherence bandwidths and multiple transmission alternatives, the overhead from feedback signaling and/or TFI may become overwhelming. Thus, what is needed is an efficient method of specifying an AMC transmission mode for a multi-carrier communication signal transmitted between a first device and a second device.